Jakob Nielsen did a short study comparing reading speeds of iPad, Kindle, and the printed book:
The iPad measured at 6.2% lower reading speed than the printed book, whereas the Kindle measured at 10.7% slower than print. However, the difference between the two devices was not statistically significant because of the data's fairly high variability.
Thus, the only fair conclusion is that we can't say for sure which device offers the fastest reading speed. In any case, the difference would be so small that it wouldn't be a reason to buy one over the other.
But we can say that tablets still haven't beaten the printed book: the difference between Kindle and the book was significant at the p<.01 level, and the difference between iPad and the book was marginally significant at p=.06.